Method for preparing microfibrillar polysaccharide

ABSTRACT

The present invention relates to a method of preparing microfibrillar polysaccharide comprising treating a polysaccharide in an aqueous suspension comprising an oxidant and at least one transition metal, mechanically delaminating said polysaccharide such that microfibrillar polysaccharide is formed. The invention also relates to microfibrillar polysaccharide obtainable by the method.

This application claims priority based on U.S. Provisional PatentApplication No. 60/694,255, filed Jun. 28, 2005.

The present invention relates to a method of preparing microfibrillarpolysaccharide, particularly microfibrillar cellulose (MFC),microfibrillar polysaccharide obtainable from said method, and the usethereof.

BACKGROUND OF THE INVENTION

Microfibrillar cellulose (MFC), the most common microfibrillarpolysaccharide, is prepared from wood fibres that have been delaminatedto small fragments with a large proportion of the microfibrils of thefibre walls uncovered.

The produced MFC has a high specific surface area that imparts strongbinding capacity in paper and fibre structures, high water retention,good stability in water dispersions as well as high viscosity.

The cellulose fibres can be delaminated to microfibrillar cellulose byenzyme treatment, especially by treatment with cellulases which isdisclosed in WO 2004/055268. However, delamination of fibres by means ofenzyme treatment is many times expensive and/or inefficient.

It would be desired to provide a method that could increase theproduction capacity in view of the prior art methods in which inter aliafibre clogging has impeded such attempts. It would also be desired toprovide a method of preparing microfibrillar cellulose with an increasedproduct consistency, i.e. to provide a suspension having a higherconcentration of microfibrils without suffering from fibre clogging,increase in temperature and/or pressure resulting in interruption of theproduction. It is a further objective to provide a method of preparingMFC having an increased surface charge and stability in view of MFCproducts known in the art. The present invention intends to provide sucha method.

THE INVENTION

The present invention relates to a method of preparing microfibrillarpolysaccharide comprising treating a polysaccharide in an aqueoussuspension comprising an oxidant and at least one transition metal,mechanically delaminating said polysaccharide such that microfibrillarpolysaccharide is formed.

The term polysaccharide include, without limitation, cellulose,hemicellulose, chitin, chitosan, guar gum, pectin, alginate, agar,xanthan, starch, amylose, amylopectin, alternan, gellan, mutan, dextran,pullulan, fructan, locust bean gum, carrageenan, glycogen,glycosaminoglycans, murein, bacterial capsular polysaccharides, andderivatives thereof, with cellulose being preferred. The polysaccharidemay be used as it is, or spinning may be used to generate or improvefibrous structure.

Cellulose is, however, the preferred polysaccharide for use in thepresent invention. Sources of cellulose for use in this inventioninclude the following: (a) wood fibres, e.g. derived from hardwood andsoftwood, such as from chemical pulps, mechanical pulps, thermalmechanical pulps, chemical-thermal mechanical pulps, recycled fibres,newsprint; (b) seed fibres, such as from cotton; (c) seed hull fibre,such as from soybean hulls, pea hulls, corn hulls; (d) bast fibres, suchas from flax, hemp, jute, ramie, kenaf, (e) leaf fibres, such as frommanila hemp, sisal hemp; (f) stalk or straw fibres, such as frombagasse, corn, wheat; (g) grass fibres, such as from bamboo; (h)cellulose fibres from algae, such as velonia; (i) bacteria or fungi; and(j) parenchymal cells, such as from vegetables and fruits, and inparticular sugar beets, and citrus fruits such as lemons, limes,oranges, grapefruits. Microcrystalline forms of these cellulosematerials may also be used. Preferred cellulose sources are (1)purified, optionally bleached, wood pulps produced from sulfite, kraft(sulfate), or prehydrolyzed kraft pulping processes, (2) purified cottonlinters, and (3) fruits and vegetables, in particular sugar beets andcitrus fruits. The source of the cellulose is not limiting, and anysource may be used, including synthetic cellulose or cellulose analogs.

According to one embodiment, the polysaccharide is treated in saidaqueous suspension and delaminated simultaneously. This makes the methodmore time-efficient without deteriorating the quality of the product.

According to one embodiment, the polysaccharide is treated prior todelamination.

According to one embodiment, the treatment is carried out at acidic orneutral pH such as from about 1 to about 8, or from about 2 to about 6,or from about 3 to about 5 for a time sufficient to facilitate thedelamination of the fibres of the polysaccharide. According to oneembodiment, the treatment of polysaccharide is performed in the absenceor substantial absence of any alkaline chemicals such as caustic soda orthe like.

According to one embodiment, the oxidant is added in an amount of fromabout 0.1 to about 5, or from about 0.5 to about 3, or from about 0.5 toabout 1.5 wt % based on the weight of polysaccharide.

A wide range of oxidants may be used of which radical generatingoxidants are preferred. Examples of such oxidants include inorganic ororganic peroxy compounds, ozone, ozonides like dimethyloxiran, halogen(e.g. chlorine or bromine) containing oxidants, oxygen. Inorganic peroxycompounds are particularly preferred and may, for example, be selectedfrom hydrogen peroxide or hydrogen peroxide generating compounds likealkali metal salts of percarbonate, perborate, peroxysulfate,peroxyphosphate or peroxysilicate, or corresponding weak acids. Usefulorganic peroxy compounds include peroxy carboxylic acids like peraceticacid or perbenzoic acid. Useful halogen containing oxidants includealkali metal chlorite, alkali metal hypochlorite, chlorine dioxide andchloro sodium salt of cyanuric acid. It is also possible to usecombinations of different oxidants. Further additives which may be addedto the aqueous suspension include mineral acids such as hydrochloricacid. The concentration of such acid preferably is from about 0.1 toabout 3, preferably from about 0.5 to about 1.5 Molar. Transition metalsin ionic form can be added to the polysaccharide fibres before, after orsimultaneously with the oxidant, for example in an aqueous solution.Examples of useful metals include iron, copper, manganese, tungsten andmolybdenum, of which iron (e.g. Fe²⁺ or Fe³⁺) is particularly preferred.The metal ions may be used in the form of salts or complexes with commoncomplexing agents such as EDTA, DTPA, phosphates or complexing agentsbased on phosphoric acid, oxalic acid, ascorbic acid, nitrite acetate,garlic acid, fulvic acid or polyoxomethalates. Further initiators whichmay be used include TAED, cyanamide and UV light. It is also possible touse combinations of different transition metals. The amount oftransition metal employed depends on the amount of oxidant employed butis in most cases from about 0.000001 to about 20 or from about 0.00001to about 5 or from about 0.0001 to about 1 wt % based on the weight ofthe oxidant.

In the case of iron ions and hydrogen peroxide the suitable amount of Feis preferably from about 0.000001 to about 20 or from about 0.00001 toabout 10 wt % based on the weight of the oxidant.

According to one preferred embodiment, the polysaccharide is treatedwith a solution of about 0.00001 to about 10 wt % FeSO₄ based on theweight of oxidant and from about 0.5 to about 1.5 wt % H₂O₂ based on theweight of the polysaccharide during 1 h at 70° C. and at pH 4.5.

The oxidant and the transition metal may be added to the polysaccharidedispersed in water, alcohol or any other suitable inorganic or organicsolvent.

According to one embodiment, the dry weight of the aqueouspolysaccharide suspension during the treatment is from about 5 to about15, or from about 8 to about 12, or from about 9 to about 11 wt %.

According to one embodiment, the temperature in the aqueous suspensionis from about 20 to about 100, more preferably from about 60 to about80° C. According to one embodiment, the microfibrillar polysaccharide isdelaminated for about 10 to about 120, or from about 20 to about 80, orfrom about 40 to about 60 minutes.

According to one embodiment, at least about 70, or at least about 80, orat least about 90 wt % of the polysaccharide is converted tomicrofibrillar polysaccharide.

As a complement to an added oxidant it is also possible to useultrasonic sound or photo- or electro Fenton reactions (in situgeneration of hydroxyl radicals by radiation or electric currents).

For purposes of the present invention polysaccharide microfibrils referto small diameter, high length-to-diameter ratio substructures which arecomparable in dimensions to those of cellulose microfibrils occurring innature. While the present specification refers to microfibrils andmicrofibrillation, these terms are here also meant to includenanofibrils (cellulosic or other).

Cellulose being the preferred polysaccharide in accordance with theinvention is found in nature in several hierarchical levels oforganization and orientation. Cellulose fibres comprise a layeredsecondary wall structure within which macrofibrils are arranged.

Macrofibrils comprise multiple microfibrils which further comprisecellulose molecules arranged in crystalline and amorphous regions.Cellulose microfibrils range in diameter from about 5 to about 100nanometers for different species of plant, and are most typically in therange of from about 25 to about 35 nanometers in diameter. Themicrofibrils are present in bundles which run in parallel within amatrix of amorphous hemicelluloses (specifically xyloglucans), pectinicpolysaccharides, lignins, and hydroxyproline rich glycoproteins(includes extensin). Microfibrils are spaced approximately 3-4 nm apartwith the space occupied by the matrix compounds listed above. Thespecific arrangement and location of the matrix materials and how theyinteract with the cellulose microfibrils is not yet fully known.

Preferably, the polysaccharide is delaminated to such an extent that thefinal specific surface area of the formed microfibrillar polysaccharideis from about 1 to about 100, or from about 1.5 to about 15, or fromabout 3 to about 10 m²/g. The viscosity of the obtained aqueoussuspension of microfibrillar polysaccharide suitably is from about 200to about 4000, or from about 500 to about 3000, or from about 800 toabout 2500 mPas. The stability, which is a measure of the degree ofsedimentation of the suspension preferably is from about 60 to 100, orfrom about 80 to about 100%, where 100% indicates no sedimentation for aperiod of at least 2 weeks (6 months).

Microfibrillar polysaccharides produced according to the presentinvention suitably have a length of from about 0.05 to about 0.9, orfrom about 0.1 to about 0.5, or from about 0.2 to about 0.3 mm.

Non-delaminated wood fibres, e.g. cellulose fibres, are distinct frommicrofibrillar fibres because the fibre length of wood fibres rangesusually from about 0.7 to about 2 mm. The specific surface area of suchfibres usually is from about 0.5-1.5 m²/g.

Delamination can be carried out in various devices suitable fordelaminating the fibres of the polysaccharides. The prerequisite for theprocessing of the fibres is that the device is capable or is controlledin such way that fibrils are released from the fibrewalls. This may beaccomplished by rubbing the fibres against each other, the walls orother parts of the device in which the delamination takes place.Preferably, the delamination is accomplished by means of pumping,mixing, heat, steam explosion, pressurization-depressurization cycle,impact, grinding, ultrasound, microwave explosion, milling, andcombinations thereof, most preferably the mechanical delamination isperformed by grinding, milling or combinations thereof. In any of themechanical operations disclosed herein, it is important that sufficientenergy is applied such that microfibrillar polysaccharide is produced.The invention also relates to microfibrillar polysaccharide obtainableby the method as disclosed herein. Preferably, the microfibrillarpolysaccharide comprises microfibrillar cellulose, most preferablymicrofibrillar cellulose derived from an unbleached pulp. Themicrofibrillar cellulose may be used in any of the various applicationsknown in the art.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the gist and scope of the present invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the claims. While the examples herebelow provide more specific details of the reactions, the followinggeneral principles may here be disclosed. The following examples willfurther illustrate how the described invention may be performed withoutlimiting the scope of it.

All parts and percentages refer to part and percent by weight, if nototherwise stated.

EXAMPLE 1

-   a) The pulp used in the experiments was a bleached sulphite pulp    from Domsjö (15% hemicellulose content) intended for paper    production.    -   1. Reference Domsjö sulphite pulp    -   2. Pre-treated Domsjö sulphite pulp: The conditions used in the        pre-treatment step were: 10% pulp consistency, 0.01% FeSO₄ based        on the weight of dry pulp, 1% H₂O₂ based on the weight of dry        pulp, 1 h at 70° C. and pH 4.5, adjusted with sulphuric acid.-   b) MFC was produced from pulp samples 1 and 2 by passing a fiber    suspension of 1% through a pearl-mill (Drais PMC 25TEX) under the    following conditions: Zirkonium oxide pearls (65% filling grade),    rotor speed of 1200 revolutions/minutes and a flow rate of 100 l/h.    Energy demand and runnability were noted during the trials.-   c) The MFC products from trial b) were passed another time through    the mill and at the same conditions except for the flow rate which    was 200 l/h.-   d) The following MFC product properties were characterized: fiber    length, viscosity, water retention value (WRV), stability, and    charge. The results can be seen in Table 1.

TABLE 1 Energy demand and characteristics of the MFC products producedfrom a bleached sulphite pulp from Domsjö (15% hemicellulose content).Ref MFC Redox MFC Ref MFC Redox MFC Passage 1 Passage 1 Passage 2Passage 2 Energy (kWh/ton) 8561 5299 12842 7924 Fiber length (mm) 0.370.24 0.28 0.25 WRV (g/g) 4.78 4.42 5.10 5.39 Viscosity (mPas) 1486 10351839 1098 Stability (%) 100 100 100 100 Z-potential (mV) −52.5 −64.9−81.3 −101.8

EXAMPLE 2

-   a) The pulp used in the experiments was an unbleached softwood kraft    pulp from Södra Cell AB intended for the production of fully    bleached pulp.    -   1. Reference Värö kraft pulp    -   2. Pre-treated Värö kraft pulp: The conditions used in the        pre-treatment step were: 10% pulp consistency, 0.01% FeSO₄ based        on the weight of dry pulp, 2% H₂O₂ based on the weight of dry        pulp, 1 h at 70° C. and pH 4.5, adjusted with sulphuric acid.-   b) MFC was produced from pulp samples 1 and 2 by passing a fiber    suspension of 1% through a pearl-mill (Drais PMC 25TEX) under    following conditions: Zirkonium oxide pearls (65% filling grade),    rotor speed of 1200 revolutions/minutes and a flow rate of 100 l/h.    Energy demand (see Table 2) and runnability were noted during the    trials.-   c) The MFC products from trial b) were passed another time through    the mill and at the same conditions except for the flow rate which    was 200 l/h.-   d) The following MFC product properties were characterized: WRV,    viscosity, stability and Z-potential (see Table 2)

TABLE 2 The energy demand and characteristics of the MFC productsproduced from an unbleached softwood kraft pulp from Värö. Ref MFC RedoxMFC Ref MFC Redox MFC Passage 1 Passage 1 Passage 2 Passage 2 Energy(kWh/ton) 8692 5276 12810 8042 Fiber length (mm) 0.37 0.34 0.30 0.29 WRV(g/g) 5.99 4.59 4.14 4.00 Viscosity (mPas) 2160 2302 1424 805 Stability(%) 100 100 100 100 Z-potential (mV) −49.4 −69.1 −46.0 −61.5

EXAMPLE 3

-   a) The pulp used in the experiments was a dissolving sulphite pulp    from Domsjö (5% hemicellulose content).    -   1. Reference Domsjö dissolving pulp (5% hemicellulose)    -   2. Reference Domsjö dissolving pulp (5% hemicellulose) with        addition of 0.1% carboxymethylcellulose (Akucell AF 1985, DS:        0.85 and MW: 340,000)    -   3. Pre-treated Domsjö dissolving pulp (5% hemicellulose). The        conditions used in the pre-treatment step were: 10% pulp        consistency, 0.01% FeSO₄ based on the weight of oxidant, 1% H₂O₂        based on the weight of dry pulp 1 h at 70° C. and pH 4.5,        adjusted with sulphuric acid.    -   4. Pre-treated Domsjö dissolving pulp (5% hemicellulose) as in        point 3 with addition of 0.1% carboxymethylcellulose (Akucell AF        1985, DS: 0.85 and MW: 340,000).-   b) MFC was produced from pulp samples 1 and 2 by passing a fiber    suspension of 1.5% through a pearl-mill (Drais PMC 25TEX) under the    following conditions: Zirkonium oxide pearls, 65% filling grade,    rotor speed 1200 revolutions/minutes and flow rate 100 l/h. Energy    demand and runnability were noted during the trials.-   c) The MFC products from trial b) were passed another time through    the mill and at the same conditions except for the flow rate which    was 200 l/h.

TABLE 3 Total energy consumption for producing MFC from a sulphite pulpfrom Domsjö (5% hemicellulose content). Passage 1 Passage 2 Pulp(kWh/ton) (kWh/ton) Reference pulp 5956 8934 Reference pulp + CMCaddition 4992 7626 Pre-treated pulp 3712 5692 Pre-treated pulp + CMCaddition 3941 5875Microfibrillated cellulose having an arithmetic average fiber length of0.23 to 0.37 mm, a water retention value of at least 400% and highstability have been produced. The MFC products produced from the redoxtreated pulp have a higher charge (Z-potential) compared to thereference products. The energy demand decreased by about 40% when thepulps were pretreated with the Fenton's reagent. Furthermore, therunnability of the pearl-mill was improved (no clogging, temperature orpressure stop). Similar trends in the results are obtained for thetrials with the unbleached softwood kraft pulp and bleached dissolvingpulp. The viscosity of the bleached sulphite pulp (15% hemicellulosecontent) and unbleached kraft pulp decreased by 47 to 66% when subjectedto the pre-treatment (cf. Table 4).

TABLE 4 Characteristics of the sulphite pulp (15% hemicellulose content)and unbleached kraft pulp used in the investigation. Domsjö Domsjö VäröVärö reference pulp redox pulp reference pulp redox pulp Viscosity 998531 1193 397 (dm³/kg) WRV (g/g) 1.49 1.60 1.46 1.50 Fiber length 0.910.92 1.12 1.18 (mm) Z-potential −18.2 −17.0 −19.4 −10.8 (mV)

1.-14. (canceled)
 15. A microfibrillar polysaccharide obtained by treating a polysaccharide in an aqueous suspension comprising an oxidant and at least one transition metal, mechanically delaminating said polysaccharide in such manner that it comprises microfibrillar polysaccharide lengths from at least about 0.05 mm.
 16. A microfibrillar polysaccharide according to claim 15, wherein the polysaccharide is derived from unbleached pulp.
 17. A microfibrillar polysaccharide according to claim 15, wherein the transition metal is in ionic form.
 18. A microfibrillar polysaccharide according to claim 17, wherein the transition metal is added to the polysaccharide in the form of a salt or metal ion complex.
 19. A microfibrillar polysaccharide according to claim 15, wherein said polysaccharide comprises microfibrillar polysaccharide lengths from about 0.05 mm to about 0.9 mm.
 20. A microfibrillar polysaccharide according to claim 19, wherein the microfibrillar polysaccharide lengths are from about 0.1 mm to about 0.5 mm.
 21. A microfibrillar polysaccharide according to claim 20, wherein the microfibrillar polysaccharide lengths are from about 0.2 mm to about 0.5 mm. 